Zimbra UI slow on sending emails

Hi.
Some of our users are experiencing slowness when using the UI.
For example when pressing the "Send" button it can take up to 3 or 4 seconds to get the UI responsive again.
The server has no high load when this happens.

Mailboxes are pretty big, talking about >3G. Can this matter? AFAIK Zimbra shouldn't be affected by this...
It happens either with Firefox and Chrome, Linux and Windows.